1. Field of the Invention
The present invention relates generally to object inspection apparatus and more particularly, to the use of a digital image enhancement means in an automatic photomask inspection system of the type used to locate extremely small defects in the transparencies of a photomask used to manufacture semiconductor devices.
2. Reference to Related Applications
This application is related to the subject matter disclosed in and claimed in U.S. Pat. No. 4,247,203, entitled "Automatic Photomask Inspection System and Apparatus"; U.S. Pat. No. 4,347,001, entitled "Automatic Photomask System and Apparatus"; U.S. patent application entitled "Automatic System And Method For Inspecting Hole Quality", Ser. No. 505,848, filed June 24, 1983; U.S. patent application entitled "Reticle Inspection System", Ser. No. 474,461, filed Mar. 11, 1983; U.S. patent application entitled "Photomask Inspection Apparatus And Method With improved Defect Detection", Ser. No. 492,658, filed May 8, 1983; and U.S. patent application entitled "Photomask Inspection Aparatus And Method Using Corner Comparator Defect Detection Algorithm", Ser. No. 494,762, filed May 12, 1983.
3. Discussion of the Prior Art
One of the major sources of yield loss in the manufacture of large scale integrated circuits (LSI) is random defect in the photomasks that are used to photolithigraphically manufacture the devices. Discussions of prior art problems and solutions therefor are fully disclosed in the above-identified patents and pending applications and such information is expressly incorporated herein by reference.
As inspection requirements become more demanding, e.g. requiring recognition of smaller features, detection of smaller defects, and capacity to handle higher geometry densities, successful inspection becomes contingent upon being able to operate at or near the physical limits of optical systems. For example, in order to guarantee that the system will "see" a very small defect, there must be sufficient gain through the optics and image acquisition system over a sufficiently wide band of spatial frequencies so as to provide modulation to the defect which exceeds a worst case noise threshold. The shape of the Modulation Transfer Function of the optics is determined by the limiting Numerical Aperture (N.A.) of the optical system and the mean wavelength and degree of coherency of the light used. Increasing the Numerical Aperture increases the optical bandwidth; however, it is at present extremely difficult to significantly increase the N.A. beyond the presently used 0.85 while otherwise maintaining acceptable performance. Immersion objectives can achieve higher useful N.A.'s but their use is unacceptable in this application due to the risk of introducing contaminant to the sample. Decreasing the wavelength of the light increases the bandwidth of the optics; however, the sensitivity of silicon photodiodes begins to fall rapidly at wavelengths significantly shorter than the presently used 540 nm. Other forms of photo-detectors do not offer the speed, sensitivity and high number of elements available with silicon photodiodes. Increasing the coherency of the light used can increase the optical gain at low and moderate frequencies but this also has its costs. With increased coherency comes decreased optical bandwidth, increased probability of fringing and decreased validity in treating the system as linear in intensity. Given these constraints and the need for improved modulation to small defects, non-optical techniques of modifying the system Modulation Transfer Function were explored.
4. Discussion of Solution Theory
The present invention seeks to obtain a solution by recognizing the mechanisms by which the image is degraded and then by operating on the degraded image with a digital filter so as to restore it to a substantially undegraded form. For example, in the automatic photomask inspection systems to which the invention is presently applied the degradations are due to the non-zero width of the optics point spread function, the blurring which results as a function of sampling the image on the fly, and the non-zero coupling between adjacent sensor sites on the photodiode arrays. The comdined effect of these factors may be mathematically represented in the space domain as the convolution of the undegraded image with a 2-dimensional blurring function; or, in the frequency domain as the product of the fourier transform of the undegraded image multiplied by the fourier transform of the 2-D blurring function. If a filter is introduced whose transfer function is equal to the inverse of the fourier transform of the 2-D blurring function the data at its output would represent the undegraded image. In practice the filter function must deviate from this ideal so as to maintain adequate signal-to-noise ratio at its output given finite signal-to-noise ratio at its input and finite precision on the mathematical operations carried on by the hardware. Although the ideal cannot be achieved practically, it has been demonstrated that this technique provides the necessary improvements in modulation to smaller defects to allow them to be detected reliably.
It is important to note that although the filter function may be combined with the defect detection function, the separation in hardware of these functions greatly reduces the complexity of the hardware involved and allows greater flexibility in matching the hardware performance to variations in other system performance parameters.
In the present embodiment, the filter function is accomplished through the use of a 7.times.7 Finite Impulse Response filter. This filter produces a corrected pixel of output as the weighted sum of each pixel of input and its nearest 48 neighbors. Although the filter is implemented in hardware so as to achieve very high data rates, the weightings, or coefficients, may be changed by the system software to adapt the filter to changes in other system performance parameters. Due to circular symmetry of the optics point spread function, left-right symmetry of the motion blurring, and top-bottom symmetry of the photodiode coupling phenomenon, the 2-D blurring function and correspondingly the coefficients of the inverse filter function exhibit top-bottom and left-right symmetry as well. The present implementation takes advantage of this symmetry by combining mathematical operations which could not otherwise be combined and consequently results in a reduction in hardware complexity.